Re: Enabling Checksums

On 2013-03-28 21:02:06 -0400, Robert Haas wrote:
> On Wed, Mar 27, 2013 at 10:15 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> > On 2013-03-27 10:06:19 -0400, Robert Haas wrote:
> >> On Mon, Mar 18, 2013 at 4:31 PM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
> >> > to get them going again. If the install had checksums, I could have figured
> >> > out which blocks were damaged and manually fixed them, basically go on a
> >> > hunt for torn pages and the last known good copy via full-page write.
> >>
> >> Wow. How would you extract such a block image from WAL?
> >>
> >> That would be a great tool to have, but I didn't know there was any
> >> practical way of doing it today.
> >
> > Given pg_xlogdump that should be doable with 5min of hacking in 9.3. Just add
> > some hunk to write out the page to the if (config->bkp_details) hunk in
> > pg_xlogdump.c:XLogDumpDisplayRecord. I have done that for some debugging already.
> >
> > If somebody comes up with a sensible & simple UI for this I am willing to
> > propose a patch adding it to pg_xlogdump. One would have to specify the
> > rel/file/node, the offset, and the target file.
>
> Hmm. Cool. But, wouldn't the hard part be to figure out where to
> start reading the WAL in search of the *latest* FPI?

I'd expect having to read the whole WAL and write out all the available
FPIs. You might be able to a guess a bit based on the LSN in the header.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
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